The nomenclature for cytokines, uh, is con- inconsistent with some named interleukin followed by a number, and others named for a biological activity first attributed to them, such as tumour necrosis factor, TNF, or interferon.
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I now have enough textbook content for a thorough answer. Here is the full explanation:
Cytokine Nomenclature - Why It Is Inconsistent
You are right, and Janeway's Immunobiology (10e) states this explicitly:
"In an attempt to develop a standardized nomenclature for molecules secreted by, and acting on, leukocytes, many cytokines are called by the name interleukin (IL) followed by a number... However, not all cytokines are included in this system; thus, students of immunology are still faced with a somewhat confusing and difficult task."
- Janeway's Immunobiology 10e, §3-15
The inconsistency is historical: each cytokine was named by whichever lab discovered it, based on whatever biological activity they first observed. Systematic renaming was attempted but never fully applied.
The Major Naming Systems Side by Side
| Group | Naming Convention | Why That Name | Examples |
|---|---|---|---|
| Interleukins | IL + sequential number | Attempt at standardisation; "inter-leukocyte" signalling | IL-1, IL-2 ... IL-38 |
| Interferons | IFN + Greek letter (α, β, γ) | They "interfere" with viral replication | IFN-α (type I), IFN-β (type I), IFN-γ (type II) |
| Tumour Necrosis Factors | TNF + Greek letter | Observed to cause haemorrhagic necrosis of tumours in vivo | TNF-α (TNF), TNF-β (lymphotoxin) |
| Chemokines | CXCL, CCL, CX3CL, XCL | Structural classification by cysteine motif + "L" for ligand | CXCL8 (= old IL-8), CCL2 (= MCP-1) |
| Colony-Stimulating Factors | Named for the colony they stimulate | Functional activity | GM-CSF, G-CSF, M-CSF |
| Transforming Growth Factors | TGF + Greek letter | Originally thought to "transform" normal cells | TGF-β |
The Five Structural Cytokine Families
Janeway's groups cytokines by structure, which cuts across the inconsistent naming above:
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IL-1 family - 11 members (IL-1α, IL-1β, IL-18, IL-33, etc.). Most are produced as inactive proproteins cleaved to the mature form; IL-1α is the exception (both pro- and cleaved forms are active). Receptors carry TIR domains and signal via NF-κB - the same pathway used by Toll-like receptors.
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Hematopoietin (class I cytokine) superfamily - the largest group. Includes most numbered interleukins, erythropoietin, and growth hormone. Receptors lack intrinsic kinase activity; they signal through JAK-STAT (Janus kinase - Signal Transducer and Activator of Transcription). Four JAKs exist (Jak1, Jak2, Jak3, Tyk2); loss-of-function mutations in Jak3 cause X-linked-negative SCID.
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Interferons - type I (IFN-α, IFN-β; antiviral, induced by pattern-recognition sensors) and type II (IFN-γ; produced by NK cells and T cells, activates macrophages). Also signal via JAK-STAT but through distinct STAT combinations.
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TNF family - more than 17 members. A key structural feature is that most are transmembrane proteins (homotrimers), limiting their range of action; some can be shed. TNF-α binds TNFR-I (broad expression - endothelial cells, macrophages) or TNFR-II (mainly lymphocytes). Receptors are structurally unrelated to classes above and signal through TRAF proteins via the non-canonical NF-κB pathway.
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Chemokines - small chemoattractant cytokines classified by the spacing of N-terminal cysteine residues: CC, CXC, CX3C, XC. Their receptors have a 7-transmembrane G-protein-coupled structure. The old functional names (e.g., IL-8, MCP-1, RANTES) have largely been replaced by systematic CXCL/CCL designations - itself an attempt to impose order that added yet another layer to remember.
Why the Mess Persists
- Cytokines were discovered by independent labs over several decades, each naming their factor after its first-observed effect (e.g., "B-cell growth factor" = IL-4; "T-cell replacing factor" = IL-5).
- The IL-numbering system was introduced as a fix but was never retroactively applied to well-established names like TNF or IFN.
- Chemokines were renamed systematically in 2002, but old names remain in wide clinical and research use alongside the new ones.
- A single cytokine often has multiple names from its pre-standardisation history (e.g., TNF-α was once called cachectin; IL-8 is now CXCL8).
The net result is exactly what you noted: a partially systematic, partially historical naming scheme that requires learning on a case-by-case basis.
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